If you've managed to submit the compulsory one and you've done enough work to understand the content of the course you might as well carry on and write the exam. You never know; you might survive it. Even if you don't you'll have put in that much more toward passing it next time round when you're done.
Ah hang on, I got an sms the other day saying there's an extension to 7 October for assigments originally due between 12 and 19 September. Perhaps you fall in that category? They've disabled the assignments facility while they work on it, apparently.
So maybe double check the messages on your cellphone and see if you didn't miss one like that?
If you've got the extension, yes it is starting to look like it'd be a good idea to drop it off in town, isn't it?